But with the Poliwhirl I had (prior to resetting its happiness via trading to turn it into Politoed) Stadium said its happiness rating was "Not at all" (the 0 - 44 range). I double checked this with the Happiness Rater, who gave me the "It looks mean." response.
It's now back at 70 thanks to the reset upon trading. I'll be keeping an eye on it as I walk around with it in my party, as I won't be likely to level it up any more than where it is now. It is my HM slave for Team Breeder, but I am curious as to why it just started to hate me so suddenly.